How can Thresh work as an ADC?

His E passive is what makes it possible, which with full build and around 150-200 souls (take or give) can equal around 900-1300 damage on first AA JUST from the E passive, in magic damage, making it harder to build against.

Why is he better then the other ad carry heroes?

Because of the passive. You will have much more armor then the other ad heroes and also u will have ap. So if you buy Nashor's teeth u will kick them past! A tanky ad carry like graves but much mobile and also better teamfights!

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